The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material in the fabrication of semiconductor devices.
Low pressure radio frequency (RF) generated plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created within the plasma impact the surface of the target to dislodge, i.e., xe2x80x9csputterxe2x80x9d material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths from the target to the substrate being deposited at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched trenches and holes of semiconductor devices having trenches or holes with a high depth to width aspect ratio, can bridge over causing undesirable cavities in the deposition layer. To prevent such cavities, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 1% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term xe2x80x9cdense plasmaxe2x80x9d is intended to refer to one that has a high electron and ion density.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
In many high density plasma applications, it is preferable for the chamber to be operated at a relatively high pressure so that the frequency of collisions between the plasma ions and the deposition material atoms is increased to increase thereby the resident time of the sputtered material in the high density plasma zone. However, scattering of the deposition atoms is likewise increased. This scattering of the deposition atoms typically causes the thickness of the deposition layer on the substrate to be thicker on that portion of the substrate aligned with the center of the target and thinner in the outlying regions. It has been found that the deposition layer can be made more uniform by reducing the distance between the target and the substrate which reduces the effect of the plasma scattering.
On the other hand, in order to increase the ionization of the plasma to increase the sputtering rate and the ionization of the sputtered atoms, it has been found desirable to increase the distance between the target and the substrate. The coil which is used to couple energy into the plasma typically encircles the space between the target and the substrate. If the target is positioned too closely to the substrate, the ionization of the plasma can be adversely affected. Thus, in order to accommodate the coil which is coupling RF energy into the plasma, it has often been found necessary to space the target from the substrate a certain minimum distance even though such a minimum spacing can have an adverse effect on the uniformity of the deposition.
It is an object of the present invention to provide an improved method and apparatus for generating a plasma within a chamber and for sputter depositing a layer which obviate, for practical purposes, the above-mentioned limitations.
These and other objects and advantages are achieved by, in accordance with one aspect of the invention, a plasma generating apparatus which inductively couples electromagnetic energy from a coil which is also adapted to sputter material from the coil onto the workpiece to supplement the material being sputtered from a target onto the workpiece. The coil is preferably made of the same type of material as the target so that the atoms sputtered from the coil combine with the atoms sputtered from the target to form a layer of the desired type of material. It has been found that the distribution of material sputtered from a coil in accordance with one embodiment of the present invention tends to be thicker at the edges of the substrate and thinner toward the center of the substrate. Such a distribution is very advantageous for compensating for the distribution profile of material sputtered from a target in which the material from the target tends to deposit more thickly in the center of the substrate as compared to the edges. As a consequence, the materials deposited from both the coil and the target can combine to form a layer of relatively uniform thickness from the center of the substrate to its edges.
In one embodiment, both the target and the coil are formed from relatively pure titanium so that the material sputtered onto the substrate from both the target and the coil is substantially the same material, that is, titanium. In other embodiments, other types of materials may be deposited such as aluminum. In which case, the coil as well as the target would be made from the same grade of aluminum, i.e., target grade aluminum. In other embodiments the target can be made of a material such as Cr, Te or SiO2. If it is desired to deposit a mixture or combination of materials, the target and the coil can be formed from the same mixture of materials or alternatively from different materials such that the materials combine or mix when deposited on the substrate.
In yet another embodiment, a second coil-like structure in addition to the first coil, provides a supplemental target for sputtering material. This second coil is preferably not coupled to an RF generator but is instead biased with DC power. Although material may or may not continue to be sputtered from the first coil, sputtered material from the coils will originate primarily from the second coil because of its DC biasing. Such an arrangement permits the ratio of the DC bias of the primary target to the DC bias of the second coil to be set to optimize compensation for non-uniformity in thickness of the material being deposited from the primary target. In addition, the RF power applied to the first coil can be set independently of the biases applied to the target and the second coil for optimization of the plasma density for ionization.